Fine-resolution, high-performance synthetic aperture radar (SAR) system having real-time image formation capabilities are currently being developed. For example, a system being developed by Sandia National Laboratories has a 4 GHz first intermediate frequency (IF) receiver. A simplified block diagram of the current generation IF receiver is shown in FIG. 1 which has been labeled as prior art.
The first IF is currently translated into a 500 MHz 2nd IF 115, which facilitates the use of surface-acoustic wave (SAW) filters 120 for band-limiting the IF. The filtered 2nd IF is then quadrature converted 125 to base-band video (I and Q channels) prior to analog-to-digital conversion at analog-to-digital converters (ADCs) 130. IF translations, filtering and analog quadrature demodulation are performed in the receiver module. The digitization and some front-end digital signal processing (DSP) (presumming and high-pass filtering) is performed in the analog-to-digital converter (ADC) modules 130.
The primary disadvantages of an analog receiver implementation are:                The IF filtering is performed in expensive and it is hard to find SAW filters 120 with limited pass-band and stop-band performance,        A triple conversion receiver is required due to the need of a 2nd IF frequency to accommodate the SAW filters,        The analog quadrature demodulation circuit requires calibration, component screening, tuning, or all of the above to maintain adequate quadrature image rejection,        Two balanced ADCs 130 are required, and        The implementation has very little operational flexibility.        
What is needed is an improved system or module for receiving IF. The present inventors have found a way to overcome the limitations of analog receivers in SAR systems with use of a digital IF receiver.